The present invention relates to a gate dielectric film forming method. More specifically, the invention relates to a gate dielectric film forming method of forming gate dielectric films for semiconductor electron devices, particularly for metal insulator semiconductor field effect transistors typified by MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
As a method of fabricating gate dielectric films for MOSFETs of VLSIs or the like, there has been a method which comprises steps of: performing a first oxidation of a silicon (Si) substrate in an oxidizing gas ambient containing no nitrogen to form a first silicon oxide film (SiO.sub.2) on a surface of the silicon substrate; thereafter, performing oxynitridation of the silicon substrate in a nitrogen-oxygen compound gas ambient (NO, N.sub.2 O, NO.sub.2, etc.) to form an oxide film containing nitrogen (hereinafter, referred to as "oxynitride film") between the first silicon oxide film and the silicon substrate; and further performing another oxidation (re-oxidation) in an oxidizing gas ambient containing no nitrogen to form a second silicon oxide film between the oxynitride film and the silicon substrate (E. Hasegawa et al., International Electron Devices Meeting Technical Digest, pp. 327-330, 1995). Forming the second silicon oxide film between the oxynitride film and the silicon substrate causes rearrangement of atoms to occur. As a result, it becomes possible to solve various problems caused by oxynitridation, such as a strain in the Si/SiO.sub.2 interface, an increase in hole traps due to chemical species related to nitrogen such as Si.sub.2 .dbd.NH, or a decrease in the carrier mobility in regions having low electric fields in directions vertical to the MOS interface.
However, when merely the above method is executed, the film thickness uniformity or the interface flatness of the gate dielectric film is deteriorated by re-oxidation, resulting disadvantageously in degraded dielectric breakdown properties of the gate dielectric film (e.g., J. Kim et al., IEEE Electron device Letter, vol. 14, No. 5, pp. 265-267, 1993).
This mechanism can be considered as follows. Here, FIGS. 10A-10D are used to explain the mechanism.
In order to perform the first oxidation, a silicon substrate (indicated by 201 in FIG. 10A) is washed into a clean state free of naturally oxidized films. However, before the silicon substrate 201 is placed in an oxidizing furnace, particularly on its way to be introduced into an oxidation reaction tube, the silicon substrate 201 is subjected to exposure to the air at high temperature. As a result, a non-uniform, naturally oxidized film 202A may be formed on the surface of the silicon substrate 201, as shown in FIG. 10A, so that roughnesses may be generated at an Si/SiO.sub.2 interface 201a. In this state, the first oxidation is performed so that a silicon oxide film 202 of a specified film thickness is formed as shown in FIG. 10B, with the result of increased roughnesses of the Si/SiO.sub.2 interface 201a. Next, when the oxynitridation is performed, indeed the interface 201a between an oxynitride film 203 and the silicon substrate 201 may become smoother than before the oxynitridation process is performed, as shown in FIG. 10C, but there will arise non-uniformities in the film thickness or nitrogen concentration of the oxynitride film 203 due to scattering of film thickness of the original silicon oxide film 202. Subsequently, when the re-oxidation is performed to form a silicon oxide film 204 between the oxynitride film 203 and the silicon substrate 201, as shown in FIG. 10D, the diffusion of oxygen molecules from the ambient into the silicon substrate 201 may be blocked by the presence of nitrogen atoms in the oxynitride film 203. As a result, the degree of scattering of film thickness of the silicon oxide film 204 may become larger and the roughness of the interface 201a between the silicon oxide film 204 and the silicon substrate 201 may also increase. This will cause a wide scattering of film thickness of the gate dielectric films 202, 203 and 204 as a whole, and which in turn cause a decrease in the dielectric breakdown withstand voltage of the gate dielectric films.
Thinning the film thickness of the silicon oxide film 204 formed by re-oxidation and/or shortening the re-oxidation time prevents any lowering of the dielectric breakdown withstand voltage, but various problems related to the oxynitridation will remain unsolved.